Elephants have been described as landscape engineers and, above certain population densities, they make considerable impacts upon the habitats in which they reside. In certain highly sensitive environments such impacts can arise at densities of as little as 0.1/sq km. However, more typically, densities of 0.3- 0.5 were regarded in the past as being acceptable in semi-arid environments. On better watered and more fertile ground, even higher densities were not considered harmful. (However, more productive land has now largely been taken for agricultural use or will soon be demanded for such in consequence of burgeoning human numbers.) In the past, impact upon habitat was regarded as synonymous with damage to habitat and, as densities progressed well above the limits at which impacts started, they were regarded as progressively damaging until reaching the stage of irreversibility. These past judgements (e.g. Graham Child, 2004 www.archive.kubatana.net/docs/wild/graham_child_elephant_culling_0401.pdf ) are now being called into question by a new breed of conservationist which tends to be inimical to any direct human control over elephant numbers, preferring, instead, for nature to be allowed to take its own course. Such conservationists tend to argue that habitats can never be stable and that their modification by elephants should not necessarily be regarded as damaging (www.ceru.up.ac.za/elephant/faqs.php )
Whether damaging or not, there is a degree of consensus over the nature of habitat modification effected by elephants. Under optimum conditions, unaccompanied by culling, hunting or poaching, elephant numbers will increase annually at approximately 5.5%/annum, doubling every 13 years until lack of adequate nutrition causes numbers to stagnate or, in the event of severe drought, to crash. In semi-arid regions, elephants principally eat grass and herbs in wet seasons and the leaves, bark and twigs of woody plants in the dry season. This is because the digestibility of grass is superior in the wet season but falls below that which is maintained by woody plants in the dry season.
Elephants need access to surface water, both for drinking and bathing and are seldom to be found further than 8km from it. Thus, the most obvious habitat modification wrought by elephants manifests as changes in tree cover within 8 km of water and this principally occurs during the dry season. However, during the wet season, elephants at high density will compete for resources with other grazers and mixed feeders. Furthermore, heavy grass grazing pressure by any herbivore will be likely to alter the species mix of the sward and will tend to lower the diversity of the plants therein.
Elephants and several other herbivores are fairly non-selective in their grazing habits and won’t suffer from simpler swards. The same doesn’t apply to selective grazing species. Typically, in natural environments, an excess of mesoherbivores and even buffalos is checked by wild predators. However, elephant numbers are not controlled in this way unless man adopts the role of predator.
The most visually obvious habitat modifications effected by elephants relate to changes in ratios of trees, shrubs and grasses. These ratios will differ between ecosystems according to soil types and rainfall levels, which largely determine the species composition of the woody plants that grow in them. Some species are destroyed if heavily browsed while others respond by coppicing. In the former case, trees give way to grass. In the latter, trees with normal habit are replaced by shrub cover composed of the same and additional species and grassland areas may diminish.
Modern elephant ecologists suggest that elephants, though capable of and, in fact, useful for landscape modification will cause no damage unless constrained by limitations of space. They suggest that land managers are largely responsible for elephants becoming problem animals, both by the erection of fences and by the provision of artificial dry season water holes, but for which elephants would disperse to “green fields and pastures new” at the start of the wet season and before their landscape modifications became damaging.
They further argue that it is wrong to suggest that population density of elephants can be related to degree of damage, correctly observing that densities will change with seasons in unconstrained populations. Heavy browsing pressure for a short period of the year, followed by recovery time, may, indeed, be less damaging than continuous moderate pressure.
Nevertheless, “elephant days of use”, which incorporates density and annual proportion of time spent in a fixed habitat, would probably be a useful metric. The “modern ecologist” seems to envisage an ideal in which elephant metapopulations roam large expanses and control their own population size by regulating their reproductive rates to match available resources. It is, of course, correct that birth rate is density dependent and that a maximum carrying capacity will eventually be reached when food or water availability become limiting.
However, this begs the question as to whether such a scenario is necessarily desirable. What, in the meantime, happens to other plant and herbivore species in the ecosystem and, given elephant longevity and lack of predators, is it not possible that there could be population “overshoot” with associated irreversible habitat damage involving topsoil losses and desertification? The answers to these questions are needed before any decision can be made as to whether the “traditional” or “modern” conservationists are correct about the potentially damaging effects of too many elephants. It is, therefore, worth examining the subject in more detail below:
- The modernist will argue that elephant populations were obviously far larger in the past before large scale hunting for ivory with modern weaponry reduced them. This would suggest that that elephants must have been capable of self-limiting in the past before any onset of ecosystem damage. Why, therefore, should the same not apply now? As numbers in some regions recover and approach assumed past levels, the population can be expected to self-limit before ecosystem damage occurs, provided, of course, that metapopulation ranges aren’t constrained.
- The traditionalist will suggest that, in areas of Botswana and Zimbabwe, for example, elephant densities are probably far greater now than they have ever been. Some evidence exists that man, acting as the major keystone predator, has been controlling elephant numbers for at least 10 000 years. Lack of any such controls is a very modern development which will have (or well could have) adverse repercussions for habitats that will prove very difficult or impossible to reverse.
It seems probable that both “traditional” and “modern” conservations would agree that Hwange National Park provides a good example of a park which is overstocked with elephants to the detriment of other species and, again, they would probably agree that this is mostly caused by the provision of pumped water during the dry season (a man-made problem).
In the northern and eastern parts of the park where most water is pumped, elephant densities are in the region of 4/sq km throughout most of the year. In the Park as a whole, the mean dry season density is around 2.5- 3. M.Valeix et al (2007) found that increasing elephant abundance in the northern part of the park which they surveyed was associated with falling numbers of all other mammalian herbivore species (www.jstor.org/stable/4499072 ).
Interestingly, these authors saw no associated changes in proportions of open areas, shrub cover and tree cover at the landscape scale. However, their study only began some 40 years after elephant densities first started to rise in consequence of pumped water. This might suggest that the landscape scale changes had already occurred and had subsequently stabilised before the study started. However, it could have been associated with a considerable loss of plant species heterogeneity, something that could not have been be picked up by the methodology used in the study. Further, open areas were not split between those that were bare (with possible loss of top soil ) and those vegetated. At the end of the study, over half of all individual grazing mammals were elephants and they made up around 90% of the biomass.